Influenza, commonly known as "the flu," is a highly contagious viral infection of the respiratory tract.

Immunization Schedule

These groups, who are at increased risk of flu-related complications, should receive the flu shot every year:

all children between 6 months and 18 years old, especially those 6 to 59 months old

any child or adult with chronic medical conditions, such as asthma, cystic fibrosis, diabetes, sickle cell anemia, and HIV/AIDS

children and teens on long-term aspirin therapy

anyone age 50 and older

women who will be pregnant during the flu season

anyone who lives or works with infants (especially those under 6 months old)

residents of long-term care facilities, such as nursing homes

health care personnel who have direct contact with patients

out-of-home caregivers and household contacts of anyone in any of these high-risk groups

In the past, there have been times when there were vaccine shortages and delays. So talk with your doctor about availability.

For kids younger than 9 who are getting a flu shot for the first time, it's given in two separate shots a month apart. It can take about 2 weeks after the shot is given for the body to build up protection to the flu.

Another non-shot option called the nasal mist vaccine came on the market in 2003 and is now approved for use in healthy 2- to 49-year-olds. But this nasal mist isn't for everyone, and can't be used by high-risk children and adults or pregnant women.

Why the Vaccine Is Recommended

The flu vaccine reduces the average person's chances of catching the flu by up to 80% during the season. Getting the shot before the flu season is in full force gives the body a chance to build up immunity to, or protection from, the virus.

The shot usually becomes available between September and mid-November. Although you can get a flu shot well into flu season, it's best to try to get it earlier rather than later, if your doctor thinks it's necessary. However, even as late as January there are still 2 to 3 months left in the flu season, so it's still a good idea to get protection.

Even if you or your child got the vaccine last year, that won't protect you from getting the flu this year, because the protection wears off and flu viruses constantly change. That's why the vaccine is updated each year to include the most current strains of the virus.

Possible Risks

Given as one injection in the upper arm, the flu shot contains killed flu viruses that will not cause someone to get the flu, but will cause the body to fight off infection by the live flu virus. Getting a shot of the killed virus offers protection against that particular type of live flu virus if someone comes into contact with it.

Some of the most common side effects from the flu shot are soreness, redness, or swelling at the site of the injection. A low-grade fever and aches are also possible. Because the nasal spray flu vaccine is made from live viruses, it may cause mild flu-like symptoms, including runny nose, headache, vomiting, muscle aches, and fever. Very rarely, the flu vaccine can cause serious side effects such as a severe allergic reaction.

When to Delay or Avoid Immunization

People who should not get the flu shot include:

infants under 6 months old

anyone who's severely allergic to eggs and egg products because the ingredients for flu shots are grown inside eggs. Tell the doctor if your child is allergic before he or she gets a flu shot.

anyone who's ever had a severe reaction to a flu vaccination

anyone who's had Guillain-Barré syndrome (GBS, a rare medical condition that affects the nerves) within 6 weeks of getting a flu shot

anyone with a fever

Caring for Your Child After Immunization

Pain and fever may be treated with acetaminophen or ibuprofen. Check with your doctor about the appropriate dose. Some doctors recommend a dose just before the immunization. A warm, damp cloth or a heating pad also may help minimize soreness. Moving or using the limb that has received the injection often reduces the soreness as well.